1. Field of the Invention
The present invention relates to a control system for a plant, and particularly to a control system which controls a plant in a feedback manner so that a controlled output of the plant coincide with a target value, and performs a hold operation of a control input to the plant when a control deviation is comparatively small.
2. Description of the Related Art
Japanese Patent Laid-open No. 2005-30241 (JP '241) discloses a control system for a variable valve-actuating mechanism which continuously varies a lift amount of an intake valve of an internal combustion engine. According to this control system, a feedback control is performed so that an actual operating angle of a control shaft for changing a lift amount is made to coincide with a target operating angle. When an absolute value of a deviation between the actual operating angle and the target operation angle (this deviation will be referred to as “control deviation”) is greater than a predetermined value, a feedback control input is calculated by the normal PID (Proportional, Integral, and Differential) feedback control. When the absolute value of the control deviation is equal to or less than the predetermined value, calculation of the feedback control input is inhibited, and the integrating operation of the control deviation is stopped.
Further, JP '241 describes a technique in which the integrated control input is output and electric power is supplied to an actuator even after the control deviation becomes “0”.
The control system shown in JP '241 is configured on the assumption that the controlled object has a torque transfer mechanism which permits a torque transfer from an output shaft of the actuator to the control shaft, but inhibits a torque transfer from the control shaft to the output shaft of the actuator. According to such a torque transfer mechanism, it is possible to stop supplying electric power to the actuator when the absolute value of the control deviation is equal to or less than the predetermined value. That is, if such a torque transfer mechanism is not adopted, it is necessary to supply electric power to the actuator in order to maintain the control deviation at “0”.
When supplying electric power to the actuator in order to maintain the control deviation at “0”, there may be a case wherein the power to be supplied to the actuator can be set to a value within a predetermined allowable range, depending on characteristics of the controlled object. For example, there is a case wherein the electric current to be supplied to the actuator which is necessary to start rotation of the control shaft from a stopped state, can be set to a value within the predetermined allowable range because of a great friction between the control shaft and other components in contact with the control shaft. In such case, it is preferable that the hold current supplied to the actuator for maintaining the control deviation at “0” is set to a value as small as possible. However, if the output of the feedback control device is changed so that the hold current becomes such a small value, a problem may be caused that a time period necessary for making the actual operating angle converge to the target operating angle becomes longer, when the target operating angle changes.